The present invention relates generally to an image sensor forming method and, more particularly, to an image sensor forming method capable of defining a connection window structure by etching a field oxide.
An image sensor is an apparatus for converting optical information of one or two dimensions, to an electrical signal. Image sensors may be provided with a pick up tube and a solid state imaging device, for example. The pick up tube has been developed and applied in areas of measurement, control and recognition that employ image processing techniques and can be similar to what is used in the television industry. There are two kinds of devices commercially available as the solid state imaging device, MOS-based (Metal-Oxide-Semiconductor) detectors and CCDs (charge coupled devices).
The CMOS image sensor is a device for converting an optical image to an electrical signal, by using a CMOS manufacturing technique. CMOS image sensors employ switching schemes for detecting sequential outputs by using numerous MOS transistors per unit pixel. The CMOS image sensor is simpler to drive and more versatile in scanning than the widely used conventional CCD image sensor. Furthermore, the CMOS image sensor can integrate a signal processing circuit on a single chip, resulting in a smaller product of lower cost and reduced power consumption.
FIG. 1 is a circuit diagram showing schematically a unit pixel detection structure of a conventional CMOS image sensor, in which the unit pixel comprises a photo diode PD for photo sensing and 4 NMOS transistors. A transfer transistor Tx transports photo charge to a floating diffusion region; a reset transistor Rx draws out the charge at the floating diffusion region for signal detection; a drive transistor Dx acts as a source follower; and a select transistor Sx is for switching and addressing. In FIG. 1, Cf represents the capacitance of the floating diffusion region and Cp represents the capacitance of the photo diode.
The operation of the image sensor unit pixel above will now be described. At first, during unit pixel reset, the reset transistor Rx, the transfer transistor Tx and the select transistor Sx are turned on. At this time, the photo diode PD starts to be depleted so that carrier charging occurs at the capacitance Cp and carrier charging builds up to a voltage VDD at the capacitance Cf in the floating diffusion region. After turning off the transfer transistor Tx, the reset transistor Rx is turned off, while the select transistor Sx is left on. During this operation after an output voltage V1 from the unit pixel output Out is read and buffered, the carriers of the capacitance Cp, are transported to the capacitance Cf by turning on the transfer transistor Tx. After another output voltage V2 from the output Out is read, an analog data value corresponding to V1-V2 is converted to a digital data value.
FIGS. 2A to 2D show processing sectional diagrams for a conventional method of forming CMOS image sensors. Hereinafter, problems associated with the conventional CMOS image sensor manufacturing method will be described, with reference to these figures.
As shown in FIG. 2A, a field oxide 21 is formed on a p-type silicon substrate 20 and a gate insulation film 22 and a gate electrode 23 of the transfer transistor are additionally formed, as shown.
Next, as shown in FIG. 2B, a first photo resist pattern PR1 is formed for forming an n-type impurity ion injection region of the photo diode PD into the p-type silicon substrate 20. The incident n-type dopant, or impurity, is shown by arrows.
Next, as shown in FIG. 2C, after oxide film spacers 25 are formed on side walls of the gate electrode 23, such as by eliminating the first photo resist pattern PR1 and etching an oxide film layer placed on the p-type silicon substrate 20, a second photo resist pattern PR2 for exposing the p-type silicon substrate 20 and the previously formed n-type silicon substrate 24 is formed. A p-type impurity is then injected.
From this p-type impurity ion injection processing, as shown in FIG. 2D, a first p-type impurity ion injection region 26 is formed with a top side exposed on the upper surface of the p-type silicon substrate 20 and a bottom side in contact with the n-type impurity ion injection region 24 is formed. This p-type region 26 constructs the photo diode along with the n-type impurity ion injection region 24. A second p-type impurity ion injection region 27 electrically connecting the p-type impurity ion injection region 26 and the p-type silicon substrate 20 is formed by ion-injecting p-type impurity into the p-type silicon substrate 20 between the leftmost field oxide 21 and the n-type impurity ion injection region 24.
The second p-type impurity ion injection region 27 serves as a connection window for matching the potentials of the first p-type impurity ion injection region 26 and the p-type silicon substrate 20.
In order to form the second p-type impurity ion injection region 27, the n-type impurity ion injection region 24 and the leftmost field oxide 21 of the photo diode should be spaced by a width equal to that of the second impurity ion injection region 27. This detrimentally results in a smaller n-type impurity ion injection region 24, which leads to reduced charge capacity of the photo diode and a reduced saturation level of the CMOS image sensor.
Additionally, during formation of the n-type impurity ion injection region 24, if the first photo resist pattern PR1 is mis-aligned, then the width of the second p-type impurity ion injection region 27 is affected. In fact the second p-type impurity ion injection region 27 may not be formed at all depending on the mis-alignment, in which case the first p-type impurity ion injection region 26 and the n-type impurity ion injection region 24 would not operate as photo diode, but only as a simple p-n junction.
In accordance with an aspect of the present disclosure, there is provided a method for manufacturing an image sensor including a photo diode having: (a) a first region formed with a first dopant type, the first region being formed within a semiconductor substrate having a second dopant type, and (b) a second region formed with the second dopant type, whereby a top side of the second region is exposed on the surface of the semiconductor substrate, the method comprising the steps of; forming a field oxide on the semiconductor substrate; etching selectively the field oxide in contact with the photo diode to create an exposed portion between the etched field oxide and the photo diode region; and forming a connection window region for connecting the second region of the photo diode to the semiconductor substrate by ion-injecting the second dopant into the semiconductor substrate defined by the exposed portion.
In accordance with another aspect of the present disclosure, there is provided a method for manufacturing an image sensor including a photo diode for photo sensing and a transfer transistor for transferring photo charge from the photo diode to a signal processing region, the method comprising the steps of: (a) forming a field oxide on a semiconductor substrate having a first dopant; (b) forming a first region of the photo diode by ion-injecting a second dopant selectively into the semiconductor substrate between the field oxide and the transfer transistor; (c) etching selectively the field oxide in contact with the photo diode to expose a portion of the semiconductor substrate between the etched field oxide and the photo diode region; (d) forming the second region of the photo diode by ion-injecting the first dopant into the first region of the photo diode, such that an upper surface of a top side of the second region is exposed on the semiconductor substrate and a bottom side of the second region is in contact with the first region of the photo diode; and (e) forming a connection window region connecting the second region of the photo diode to the semiconductor substrate by ion-injecting the first dopant into the exposed portion of the semiconductor substrate.